This invention concerns droplet coalescence control elements, and relates to the phenomenon whereby when droplets come into contact with an impervious material surface which is wettable by the liquid of the droplets, each droplet tends to spread on that surface as a film so that coalescence of adjacent droplets will occur. It is here assumed that the natures of the material surface and liquid are such that they are not compatible in the sense of being chemically or otherwise mutually reactive.
Among the factors which determine the degree of wettability of a material surface by a liquid is the surface energy level of the material which requires to be high for some liquids and low for others.
It can be shown that if droplets of the dispersed phase of a liquid dispersion are directed at a perforated plate which is non-wetted by the liquid of the dispersed phase and the ratio of hole size to droplet size is less than 0.4/1, then generally droplets will not pass through the holes and there will be a build-up of the dispersed phase on the up-stream side of the plate. However, if the dispersed phase liquid wets the plate surface, then the droplets form on the plate a film which can flow through the perforations and collect and coalesce on the downstream face of the plate to become detached therefrom either as large drops or by streaming in dependence upon the volumetric flow; the plate thus acts as a droplet coalescence control element.
The significance of the surface energy level of the surface material of the control element can be demonstrated as follows. If in the above mentioned arrangement the dispersed phase is water in a kerosene/water dispersion and the surface material of the perforated plate has a high surface energy level, e.g. stainless steel, there will be effective coalescence of the dispersed aqueous phase, but if the surface material has low surface energy level, e.g. polypropylene, coalescence of the dispersed aqueous phase will not occur and it will build up on the upstream side of the plate, i.e. flooding will occur. Conversely, if the kerosene is the dispersed phase, the surface material of the plate must have a low surface energy level for coalescence to occur.
Droplet coalescence control elements are applicable in many forms for a variety of purposes. In a fluid contacting process a liquid can be caused to travel in one direction, for instance downwards in a column, as a vapour moves in the opposite direction, and appropriately formed coalescence control elements are provided in the flow path to establish large exposed surfaces of fluid whereby to aid enrichment of the vapour by the liquid. In a de-entrainment operation for removing droplets of liquid from a vapour, the droplet laden vapour is caused to flow in a path containing coalescence control elements which will be selectively wetted by and thus serve to trap the droplets, whereby the droplets will be removed from the vapour. In a de-entrainment operation for removing liquid droplets from another immiscible or partly miscible liquid, the fluid to be treated is caused to flow in a path including appropriate droplet coalescence control elements whereby the droplets are coalesced and removed. Also, in the separation of liquid/liquid dispersion by the use of a settling tank, which may be part of an operation for separating a liquid/liquid mixture by solvent extraction, there may be used droplet coalescence control elements, usually in the form of packings as hereinafter defined, to affect the dispersed phase in the sense to accelerate separation. Where reference is made to fluid being caused to flow in a path, the direction of the path may or may not be determined by the nature of the fluid; it may necessarily be vertical in some cases or horizontal in others, or in some instances it may conveniently be either.
Typical circumstances in which droplet coalescence control elements may be used are distillation columns, rectification columns, absorption towers, liquid extraction equipment, and liquid separators, particularly settlers for separating the phases of dispersions. The physical forms of the elements will be determined by the particular nature of the operations to be performed. For instance, the elements may be in the nature of baffles so that coalescence occurs on a substantially flat surface, which may or not be perforated. In some cases the elements may be serving as, or as parts of, condensers for vapours, in which case the liquid collects by coalescence and is removed in any convenient manner from a baffle surface, ordinarily a cooled surface.For many purposes, for instance filtration and separation processes, it is convenient to use a droplet coalescence control element in the form of a perforate packing, that is a body exhibiting a labyrinth formation of intersticial passages extending between an entry face and an exit face of the body. Such a perforate packing may be built up of tubular, ringlike, saddle shaped and/or platelike units, but more conveniently it can be formed of woven fabric or knitted mesh fabric, in the latter case being, for example, of the kind known in Great Britain as "knitmesh D.C." packing.
In previous proposals the surface of a droplet coalescence control element has consisted of a single material, that is a material having a uniform surface energy level which is selected according to the nature of the liquid to be controlled by it. The performance of such droplet coalescence control elements, in relation to liquid extraction and phase separation, has been explained by G. A. Davies and G. V. Jeffreys in "Filtration and Separation", Volume 6, No. 4, pages 349-354.
However there have sometimes been experienced difficulties in obtaining a control surface which is desirably compatible with a component fluid being processed.